1. Field of the Invention
The present invention relates to a method for investigating the surfaces of samples at nanometer and picosecond resolution and to a scanning tunneling microscope type device for performing this method. In particular, the invention relates to scanning tunneling microscopes in which the scanning operation is assisted by optical methods to make it faster than customary with state-of-the-art scanning tunneling microscopes. A typical application of the invention is testing integrated circuits for manufacturing quality and, hence, usability in computer or control environments.
2. Background Art
Since its invention in 1979, the Scanning Tunneling Microscope (STM) has evolved as a powerful tool for the investigation of surface structures down to the atomic level. The basic scanning tunneling microscope is described in U.S. Pat. No. 4,343,993. Briefly, a sharply pointed, electrically conductive tip is placed at a distance on the order of one nanometer from the (conductive) surface of the sample to be investigated, with an appropriate potential applied across the gap between the tip and surface. As the electron clouds of the atoms at the apex of the tip and at the surface touch, a flow of electrons will result giving rise to a tunneling current which happens to be extremely sensitive to changes in gap width. To render these changes as close as possible to zero, a feedback system controls the distance of the tip from the surface, using the deviations of the tunneling current from an initial value as a control signal. This control signal is also employed to generate a plot of the topography of the surface being investigated.
A steadily increasing demand for information relative to dynamic processes near and at surfaces and interfaces has developed in surface science, especially in the field of heterogeneous catalysis and in the development of ultra-fast devices. The only known possibility for achieving a time resolution in the picosecond or femtosecond range is given by optical methods based on pulsed laser excitation or by the inherent time-structure of synchroton radiation. These methods normally do not rely on spatial resolution.
The spatial resolution achievable with purely optical methods is based either on the focusing of an incident optical beam or on the image resolution of special electron optics. Examples of disclosures of the relevant techniques are European Patent Application EP-A 0 180 780, M. D. Jones, G. A. Massey, D. L. Habliston, and O. H. Griffith, Laser Excitation in Photoelectron Microscopy, Proc. 1st Int. Conf. on Electron Emission Microscopy, Tubingen, Sept. 1979, pp. 177-182, and G. A. Massey, M. D. Jones, and J. C. Johnson, Nonlinear Photoemission for Viewing Guided or Evanescent Waves, IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June, 1981. With these techniques, the spatial resolution remains limited to about 0.1 .mu.m which is insufficient, e.g., for the analysis of hot-electron transport in modern ultra-fast devices or multi-quantum-well structures. There, a spatial resolution of better than 10 nanometers is required.
Therefore, it is an object of the present invention to provide a technique and apparatus for the investigation of dynamic, ultra-fast surface phenomena with femto- to picosecond time resolution and with a spatial resolution down to the atomic level.
It is another object of the present invention to provide a technique and apparatus for combining high speed optical methods with STM - type devices to obtain enhanced time and spatial resolution in the analysis of rapidly occurring events.